The U.S. Postal Service announced Wednesday morning that it was moving from six- to five-day delivery to help reduce its ever growing budget deficit. But for years, the post office has argued that it needed congressional authorization to do so. So is the post office’s proposal even legal?

It’s no secret that the USPS is a financial mess. Last year it lost $15.6 billion and is facing annual budget deficits of $21 billion by 2016. One way the Postal Service has been trying to reduce those massive losses is to scale back the number of days it delivers mail. On Wednesday, Postmaster General Patrick Donahoe said the post office would no longer deliver or process first-class mail on Saturdays — though it would continue to deliver packages and keep post offices open six days a week.

In the past, the Postal Service — technically a quasi-governmental agency with congressional oversight — has argued that it would need explicit authorization from Congress to eliminate Saturday delivery. In fact, the USPS website still says, “Congress must elect not to renew the legislation requiring the Postal Service to deliver six days a week.”

But on Wednesday, Donahoe essentially announced that he’s doing it without the blessing of Congress. “We think we’re on good footing with this,” he said. “We think right now the opportunity exists to make the changes on our own.” In particular, Donahoe says the move is legal under the “continuing resolution” that is temporarily keeping the entire federal government from shutting down in the wake of the fiscal-cliff impasse.

Donahoe did not get more explicit than that, but a close reading of the continuing resolution suggests he’s technically correct. Since 1983 every appropriations bill Congress has passed has explicitly required six-day delivery. But Congress has yet to produce an appropriations bill for fiscal year 2013. In the meantime, the federal government is operating under the continuing resolution, which expires in March. And the continuing resolution does not include any language requiring six-day delivery.

So it appears that Donahoe found a window in which the post office can change its delivery schedule without congressional authorization. He’s essentially daring Congress to mandate six-day delivery in the weeks leading up to the continuing resolution’s expiration date, March 27.

But in doing so, Congress would have to ignore the fact that eliminating Saturday delivery will save the Postal Service $2 billion annually and that the American public apparently supports the move. The postmaster general cites polls showing that 70% of people say they support five-day delivery.